1. Field of the Invention
RAM coatings contain magnetic particles incorporated into a binder, such as urethane The thickness and loading of the coating must be controlled in order to obtain the proper radar absorption properties. One approach is to use a hand held thickness measuring device as disclosed in U.S. Pat. No. 5,012,248 “Radar Absorption Material Thickness Measuring Device” by J. R. Munroe, et al. This invention comprises of a radiating element assembly for transmitting RF energy to and recovering reflected RF energy from the coating. A visual display is provided to indicate the thickness of the coating. A portable power supply is coupled to the detector assembly making it portable. This device is highly suitable for use in checking repairs made in the field. While this device works well, it requires physical contact with the coating surface.
2. Description of Related Art
RAM coatings contain magnetic particles incorporated into a binder, such as urethane The thickness and loading of the coating must be controlled in order to obtain the proper radar absorption properties. One approach is to use a hand held thickness measuring device as disclosed in U.S. Pat. No. 5,012,248 “Radar Absorption Material Thickness Measuring Device” by J. R. Munroe, et al. This invention comprises of a radiating element assembly for transmitting RF energy to and recovering reflected RF energy from the coating. A visual display is provided to indicate the thickness of the coating. A portable power supply is coupled to the detector assembly making it portable. This device is highly suitable for use in checking repairs made in the field. While this device works well, it requires physical contact with the coating surface.
It is common to apply RAM coatings by use of hand and robotic spray techniques. Since coating thickness is critical, it is desirable to check the coating thickness prior to it curing. This would make the by J. R. Munroe, et al. device unusable because of the damage to the coating that would occur upon contact of the device onto the wet surface. This problem can be avoided by the use of radiating and receiving electromagnetic transmission horns angled toward each other. The signal from the radiating horn is directed at the surface and the return signal is received by the receiving horn. Thus the measurement is limited to relatively large areas. This prevents accurate readings of significantly curved surfaces. Furthermore, it cannot be used in confined areas such as the engine inlet ducts on aircraft.
Conventional inspection techniques, such as those which use ultrasonic techniques, are unsuitable, for radar absorption is not measured, and because ultrasound does not propagate well in loaded urethane or silicon based materials. Thus it is possible that the measured thickness of the coating may be correct, but the area may not be properly loaded with magnetic materials.
In U.S. Pat. No. 6,788,244B1 Inspection Device For Radar Absorbing Materials by K. K. Tam, an inspection device is disclosed for non-contact inspection of RAM surface coatings containing radar-absorbing materials on a conductive surface or substrate. In detail, the device includes a first circuit for transmitting an electromagnetic signal to the assembly. The first circuit includes a radio frequency (RF) source of electromagnetic radiation coupled to a waveguide made of a conductive material coupled in series to a second waveguide made of a dielectric material with their longitudinal axes aligned. A second circuit is provided for receiving the portion of the electromagnetic radiation transmitted by the first circuit reflected from the assembly. The second circuit includes a third waveguide made of a conductive material coupled in series to a fourth waveguide made of a dielectric material with their longitudinal axes aligned. The second circuit further includes a RF power detector coupled to the third waveguide. Thus an electromagnetic signal is transmitted from the first waveguide to the second waveguide on to the assembly and the portion of the electromagnetic signal reflected off the assembly is received by said fourth waveguide and transmitted to said third waveguide and to the RF power detector. The longitudinal axes of the first and second waveguides are at an acute angle to the longitudinal axis of the third and fourth waveguides. This angle is preferably 10 degrees.
The second and fourth waveguides are solid and made of a dielectric material such as Polytetrafluoroethylene. It is important to provide an impedance match between the first and second waveguides and the third and fourth waveguides, and the first and fourth waveguides to free space. This is accomplished by having the center portion of the second and fourth waveguides fit within the first and third waveguides. A portion of the second and third waveguides that extend into the first and third waveguides are tapered along their top and bottom surfaces to a relatively sharp edge at the end thereof. A portion of the waveguides on the ends extending out of the first and third wave guides are tapered along their sides to a relatively sharp edge.
The output from the RF power detector is fed to a programmable gain amplifier and thereafter to a signal digitizer. The programmable RF source and RF power detector, as well as the amplifier and signal digitizer are typically controlled by a microprocessor. The second and fourth waveguides maintain about 0.75 inch away from the surface of the assembly being inspected. Thus the device is typically mounted on a robotic arm, such that the assembly is automatically inspected, in a manner similar to the robotic spray machines used to apply the coating. Thus the inspection process is no different from other automated inspection systems. However, this device allows the coating to be inspected prior to its curing, while still in a “wet” condition. Thus any issue associated with the material and the application process can be corrected prior to the coating curing.
However, this device was designed for use on an automated inspection line where a robotic arm can be programmed to provide the proper distance and orientation from the RAM coating being inspected. It is unsuitable in its present form for use in a hand held applications for there is no way to properly establish the required distance from and orientation to the RAM coating surface without actually contacting the test surface.
In U.S. Pat. No. 5,355,083 Non Contact Sensor And Method Using Inductance And Laser Distance Measurements For Measuring The Thickness Of A Layer Of Material Over Laying A Substrate by A. R. George, et al. discloses the use of a laser to aid in the positioning of an Inductance type coating thickness measuring device. A pair of lasers is used to measure the thickness of a coating. One laser is designed such that beam passes through the coating onto the substrate wherein it is reflected back to a sensor. The second laser is designed so that its laser beam is reflected off the coating to a second sensor. A computer is used to process the two signals and thus determine the thickness of the coating. Such a system will only work when the coating is transparent to the first laser beam. It also depends upon having the laser beams in a fixed position. It would also be ineffective in a hand held device, since the two lasers will not provide sufficient feedback to obtain the proper distance from and orientation to the test surface.
In U.S. Pat. No. 5,868,840 Paint Gun Incorporating A Laser by R. J. Klein, II, et al. a pair of lasers are used to indicate the proper distance from a surface being coated by a spray gun. The lasers are positioned such that the beams become co-incident when the gun is the proper position from the surface being coated. However, it does not control the angle of the spray gun nozzle to the surface.
Thus, it is a primary object of the invention to provide a device for establishing the required distance from and perpendicularity to a RAM surface for a non-contacting hand held coating measurement system.
It is another primary object of the invention to provide a device for establishing the required distance from and perpendicularity to a RAM surface for a non-contacting hand held coating measurement system that is compact.
It is a further object of the invention to provide a device for establishing the required distance from and perpendicularity to a RAM surface for a non-contacting hand held coating measurement system that can automatically determine the proper distance and orientation to a RAM coating surface.